1. Field of the Invention
This invention relates to an optical pickup apparatus used for recording and reproducing an optical disc, and more particularly to an optical pickup apparatus that is capable of eliminating the crosstalk component of a high density optical disc effectively.
2. Description of the Prior Art
Generally, an optical pickup apparatus irradiates a light on the recording surface of an optical disc to detect the reflected light, thereby performing the information recording and reproducing operation. To this end, the optical pickup apparatus is composed of a laser diode for emitting a light, an objective lens for focusing the emitted light on the recording surface of the disc, and other optical system required for concentrating and receiving the light.
An optical disc, such as a compact disc(CD) or a digital versatile disc(DVD) having more improved recording capacity, has been commercially available. This optical disc has more enlarged use for recording and reproducing audio and video data and computer data, etc. Recently, an optical disc having much more improved recording capacity is expected owing to the development of a blue laser generating a short wavelength of light. There have been suggested several schemes increasing the numerical aperture(NA) of an objective lens or narrowing the track pitch along with the use of a light source such as a blue laser, etc. so as to enlarge a recording capacity of the optical disc. However, it has a problem in that a crosstalk is caused between the adjacent tracks in the case of narrowing the track pitch of the optical disc.
More specifically, since an optical spot irradiated so as to reproduce a certain pit P1 has a larger size than a width of the pit as shown in FIG. 1, it also is irradiated onto the pits in the adjacent tracks. As a result, a crosstalk component caused by the pits in the adjacent tracks is involved in the reproduced signal. A strategy employing a polarizing phase plate as shown in FIG. 2 has been known as one of methods for eliminating such a crosstalk component.
FIG. 2 shows the conventional optical pickup apparatus for eliminating the crosstalk component. In FIG. 2, the optical pickup apparatus includes a light source 12 for generating a light beam, an objective lens 20 for focusing a light beam from the light source 12 on the recording surface of an optical disc 22, first and second photo detectors 30 and 32 for converting a reflective light beam from the optical disc 22 into an electrical signal, a beam splitter 18 arranged among the light source 12, the objective lens 20 and the first and second photo detectors 30 and 32, a polarizing beam splitter(PBS) 26 arranged among the beam splitter 18 and the first and second photo detector 30 and 32, a polarizing phase plate 16 arranged between the light source 12 and the beam splitter 18, a first collimator lens 14 arranged between the polarizing phase plate 16 and the beam splitter 18, a second collimator lens 24 arranged between the beam splitter 18 and the PSB 26, and a sensor lens arranged between the PBS 26 and the first photo detector 30. The light source 12 generates two polarized beams having a polarizing characteristic moving perpendicularly to each other The first collimator lens 14 converts a divergent light beam progressing from the light source 12, via the polarizing phase plate 16, toward the beam splitter 18 into a parallel light beam to prevent a leakage of the light beam. The beam splitter 18 passes a light beam received via the first collimator lens 14 and the polarizing phase plate 16 in such a manner to be progressed toward the objective lens 20, and reflects a reflective light beam reflected from the recording surface of the optical disc 22 and passing through the objective lens 20 in such a manner to be progressed toward the second collimator lens 24. The objective lens 20 focuses an incident light beam from the beam splitter 18 on the recording surface of the optical disc 22. The polarizing phase plate 16 consists of two phase zones 16A and 16B having a phase difference of 180xc2x0 in the left and right phases thereof as shown in FIG. 3, and which is responsible for selectively changing a phase of the light beam received, via the first collimator lens 14, from the light source 12 in accordance with a polarizing characteristic thereof. More specifically, the polarizing phase plate 16 passes the first polarized beam in any one direction of two polarized beams emitted from the light source 12 as it is, thereby allowing the first polarized beam to be irradiated, via the collimator lens 16 the beam splitter 18 and the objective lens, on the recording surface of the optical disc 22 as a main beam MB as shown in FIG. 2A. On the other hand, the polarizing phase plate 16 passes the second polarized beam in a direction perpendicular to the first polarized beam of two beams from the light source 12 with a phase being modulated, thereby allowing the second polarized beam to be irradiated on the recording surface of the optical disc 22 as a twin-mountain shaped sub-beams SB1 and SB2 superposed at each side of the main beam MB as shown in FIG. 2A. The main beam MB in the light beams irradiated on the recording surface of the optical disc 22 in this manner is irradiated on the signal track to be accessed and is used to reproduced an information signal. On the other hand, the sub-beams SB1 and SB2 are irradiated on the adjacent tracks and is used to detect a crosstalk component included in the reproduced signal. The second collimator lens 24 plays a role to focus a parallel light beam reflected from the optical disc 22 and received via the objective lens 20 and the beam splitter 18. The PBS 26 passes the main beam MB with the first polarization component in a reflective light beams reflected from the beam splitter 18 and received via the second collimator lens 24 as it is in such a manner to be progressed, via the sensor lens 28, toward the first photo detector 30. On the other hand, the PBS 26 reflects the sub-beams SB1 and SB2 with the second polarization component in the reflective light beam in such a manner to be progressed toward the second photo detector 32. The sensor lens 28 focuses the main beam MB with the first polarization component passing through the PBS 26 as it is and being received thereto onto the first photo detector 30. The first photo detector 30 detects a main beam MB received via the sensor lens 28 and the second photo detector 32 detects sub-beams SB1 and SB2 received separately from the PBS 26, thereby converting them into electrical signals.
In other words, the first photo detector 30 detects a radio frequency signal including an reproducing signal from the main beam MB while the second photo detector 32 detects a crosstalk component in the adjacent tracks from the sub-beams SB1 and SB2. A crosstalk component detected at the second photo detector 32 is eliminated from the radio frequency signal detected at the first photo detector 30 to thereby detect a reproducing signal.
The first and second sub-beams SB1 and SB2 reflected from the adjacent tracks have a frequency characteristic different from each other. In particular, when a tilt is generated between the objective lens and the optical disc, two sub-beams SB1 and SB2 have a greatly different frequency characteristic. Accordingly, in order to eliminate a crosstalk component from the radio frequency signal detected at the first photo detector 30 effectively, it is necessary to detect the first and second sub-beams SB1 and SB2 and filter the same with a filter suitable for a frequency characteristic of each sub-beam so as to make an operation on the radio frequency signal detected from the main beam MB. To this end, the second photo detector 32 includes two photo detecting cells for detecting the first and second sub-beams SB1 and SB2. However, the above-mentioned optical pickup apparatus has a problem in that, since a distance between the first sub-beam SB1 and the second sub-beam SB2 focused onto the second photo detector 32 is too narrow, it is difficult to detect them separately at each photo detecting cell.
For instance, when a distance between the sub-beams SB1 and SB2 on the optical disc 22 is set to 0.72 xcexcm and a magnification between the light-focusing zone and the light-receiving zone is ten times, it is difficult to separate the sub-beams SB1 and SB2 effectively because a distance between the sub-beams SB1 and SB2 formed on the second photo detector 32 has only a value of 7.2 xcexcm. It may be considered as a scheme capable of enlarging a distance between the sub-beams SB1 and SB2 to increase a magnification between the light-focusing zone and the light-receiving zone in light of a fact that the distance is proportional to the magnification, but it is difficult to increase the magnification without limit. It is caused by a fact that a distance between the sub-beams irradiated onto the disc must be narrow in compliance with a track pitch reduced so as to improve the recording density of the disc. Accordingly, it is required to provide an apparatus that is capable of enlarging a distance between the sub-beams at the photo detector even though a distance between the sub-beams at the disc is narrow.
In addition, the above-mentioned optical pickup apparatus has a problem in that, when it is applied to an optical disc having a track pitch set to a relatively narrow value so as to provide the high density, it is impossible to eliminate a crosstalk component because the distance between the sub-beams grows distant relatively such that the sub-beams can not irradiated onto the adjacent tracks. In other words, the optical pickup apparatus is unsuitable for an optical disc having a trend toward a high density. Moreover, the above-mentioned optical pickup apparatus has a problem in that, since it needs a polarizing beam splitter 26 and two photo detectors 30 and 32 to detect the sub-beams, a bulk of the pickup becomes not only enlarged to deteriorate a slim type configuration thereof, but also the construction of optical elements becomes complicated to make the manufacture thereof difficult.
Accordingly, it is an object of the present invention to provide an optical pickup apparatus that is capable of separating sub-beams easily at photo detectors so as to eliminate a crosstalk.
accurately locating a sub-beam at such a position that a crosstalk component can be effectively eliminated.
A further object of the present invention is to provide an optical pickup apparatus that is capable of reducing a distance between sub-beams at an optical disc so as to eliminate a crosstalk and hence is suitable for a high-density optical disc.
A still further object of the present invention is to provide an optical pickup apparatus that is capable of providing sub-beams for eliminating a cross talk with a very simple configuration.
In order to achieve these and other objects of the invention, an optical pickup apparatus according to one aspect of the present invention includes a first polarizing phase plate arranged at a light phase in a light-focusing optical system for irradiating a light beam onto an optical disc; and a second polarizing phase plate arranged at a light phase in a light-receiving optical system for detecting a reflective light from the optical disc.
An optical pickup apparatus according to another aspect of the present invention includes first and second phase plates arranged at a light-focusing optical system and a light-receiving optical system, respectively, wherein a polarization direction of the first phase plate is perpendicular to that of the second phase plate.